Prime Minister Theresa May has told the Scottish government "now is not the time" for a second independence referendum, saying it would be unfair to ask people to vote without knowing the result of Brexit talks.

Scottish First Minister Nicola Sturgeon has called for a referendum before the end of the two years of talks set out under Article 50 of the EU's Lisbon Treaty to leave the European Union, accusing May of ignoring Scotland's demands.

But May, who would not say outright that she would block a Scottish bid to wage another referendum, said it was time to work together to win the "right deal" for Britain in the coming EU talks.

"Right now we should be working together, not pulling apart," May told British television.

Sturgeon said on Thursday that any attempt to block a second independence referendum would be undemocratic and evidence that May fears the verdict of the Scottish people.

"The Scottish government is not proposing #scotref now... but when the terms of Brexit clear and before it is too late to choose an alternative path," Sturgeon, who heads Edinburgh's pro-independence devolved government, said on Twitter.

Sturgeon said that if May refuses to agree to a referendum, "this would be undemocratic" and also prove that May's Conservatives "fear the verdict of the Scottish people."

A source in the SNP said May should not try to set conditions on the vote.

"This should be a referendum made in Scotland, with no conditions set by a (Conservative) prime minister. We have a very clear mandate beyond any doubt on this," said the source.

The Scottish Greens, another pro-independence party, said May risked boosting support for independence if she wanted to veto a decision expected to be made by Scotland's devolved parliament next week.

The call for a Scottish independence referendum, just two-and-a-half years after Scots voted to keep the union, has piled pressure on May, who is preparing for EU divorce talks while trying to satisfy eurosceptics in her ruling Conservative Party.